Ubiquinone (coenzyme Q or Q) functions in cells as a redox-active coenzyme of mitochondrial and plasma membrane electron transport, as well as an essential lipid soluble antioxidant. Human dietary supplementation with Q appears to have beneficial effects in slowing the progression of neuro- and muscle- degenerative diseases. Cells are capable of synthesizing Q, but much remains to be learned about the sites of its synthesis, mechanisms of inter- and intra-cellular transport, and the regulation and enzymology of its biosynthesis. The goals of the proposed research are to characterize the polypeptides of the Q biosynthetic pathway and to define the enzymology of Q biosynthesis. The experimental system takes advantage of nine complementation groups of Q-deficient (coq) mutants in the yeast Saccharomyces cerevisiae. The coq mutants provide the basis for the characterization of the Coq polypeptides in both yeast and mammals. Synthetic analogs of Q-intermediates provide reagents that serve both as standards in the isolation and identification of Q intermediates, and as substrates for assays of enzyme activities. Genetic and biochemical evidence indicate that synthesis of Q in yeast requires a mitochondrial Coq multienzyme complex. We propose to identify the polypeptide components, and to determine whether Q-intermediates constitute important lipid components of these complexes. A potentially important function of the Coq biosynthetic complex is to regulate the flux of Q and Q-intermediates through the pathway, a process that potentially impacts performance of the respiratory electron transport chain. We will also characterize a tenth complementation group of yeast mutants. The yeast coqIO mutant defines a unique respiratory deficiency because mitochondria exhibit the characteristic phenotype of Q-defiency, yet have normal levels of Q. This class of mutants has a defect in a Q binding protein that shares homology with START domain proteins. We will determine whether this Q binding protein functions as a chaperone in the delivery of Q from its site of synthesis to its proper location for respiratory electron transport. The experimental approach employs a combination of lipid chemistry, yeast genetics, and biochemistry to delineate the biosynthetic steps responsible for the production of Q in yeast and human cells.